1. Field of the Invention
The invention relates to composite materials useful as buoyancy devices and more particularly relates to syntactic foam materials assembled to form deep water, submergible buoyance devices.
2. Brief Description of the Prior Art
Buoyancy devices have been assembled of syntactic foam compositions prior to the present invention; see for example the descriptions given in the U.S. Pat. Nos. 3,622,437 and 3,729,756. In general, such devices comprise a syntactic polymeric resin foam matrix incorporating a large volume of hollow spheres, designated as "macrospheres" to distinguish them from the smaller hollow spheres designated "microspheres" which constitute a component of the syntactic foam composition. The macrospheres generally have diameters of from 0.5 to 6 inches and are generally made of thermoplastic, synthetic, polymeric resins reinforced with filler materials such as mica and chopped fibers of glass. The prior art macrospheres employed generally have densities of from 7.5 to 12.5 lbs/cubic foot. When encapsulated in a conventional syntactic foam matrix, the implosion pressure limit of these macrospheres is approximately doubled, allowing for the buoyancy devices made of the macrosphere/foam compositions to be usefully employed at substantial depths in the open sea, i.e. at depths up to about 4,500 feet. At deeper sites there is a likelihood of macrosphere implosion due to the higher hydrostatic pressures associated with those depths.
We have now found that implosion resistant macrospheres for use in buoyancy devices at sea depths in excess of 4,500 feet and up to at least about 12,000 feet may be fabricated from syntactic foams, preferably made-up from synthetic thermosetting polymeric resins.